1. Field of the Invention
This invention relates to an active filter circuit, and more particularly it pertains to a polar filter circuit such as low pass, high pass filter or the like which is constructed in the form of a semiconductor integrated circuit comprising multistage-connected active filters each of which can be arranged such that a damping pole occurs at a predetermined frequency.
2. Description of the Prior Art
Referring to FIG. 1 of the accompanying drawings, there is illustrated a band elimination filter (referred to as BEF hereinafter) which has conventionally been proposed as a filter circuit having a damping pole at a predetermined frequency.
The BEF of FIG. 1 comprises a low pass filter 20, a high pass filter 21, and an adder 22. A signal inputted at an input terminal is passed to the adder via the low pass filter 20 and high pass filter 21, and from an output terminal is derived an output having such frequency characteristics that a damping pole occurs at a predetermined frequency f.sub.0 so that the frequency f.sub.0 is eliminated.
As will be seen, the conventional BEF is arranged such that between the input and output terminals 1 and 2, the low pass filter 20 and high pass filter 21 are connected in series with the adder 22. However, difficulties have been experienced in an attempt to construct, through a multistage combination of such BEFs, an active filter circuit such as polar low pass or high pass filter provided in the form of a semiconductor integrated circuit or to set up a frequency at which a damping pole occurs, as desired.
Thus, it has been desired to realize a filter circuit with which a damping quantity or the frequency at which a damping pole occurs, can be set up as desired, and an active filter circuit which is arranged such that a polar filter circuit such a low pass or high pass filter provided in the form of a semiconductor integrated circuit, can be constructed by connecting in multistage the above type of filter circuit.